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DTSC Toxicity Criteria Rule for Human Health Risk Assessments

Responses to Frequently Asked Questions (FAQs)

California is a leader in protecting public health using strong scientific-based decision making.  The Department of Toxic Substances Control’s (DTSC) new regulation, Toxicity Criteria for Human Health Risk Assessments, Screening Levels, and Remediation Goals (1), continues this leadership. The Toxicity Criteria Rule protects human health by ensuring that human health risk assessments, risk-based screening levels, and risk-based remediation goals for hazardous waste and hazardous substance cleanup sites in California use the appropriate toxicity criteria specified in section 69021 of the rule. Toxicity criteria are chemical-specific numerical values that describe the potency of a chemical or contaminant (2).

The Toxicity Criteria Rule formalizes DTSC existing statewide practice regarding application of toxicity criteria at cleanup sites. The toxicity values set in the Rule protect California’s diverse population including through the use or consideration the following factors:

  • Best available science, i.e., the most current, sound scientific methods in risk assessment and toxicology, as required by H&SC 25356.1.5
  • California’s environmental justice laws and policies
  • Age and race sensitivities to chemical exposure
  • Exposure due to cultural and income level differences
  • Children’s special vulnerabilities to exposure to environmental hazards as required in the California’s Children’s Environmental Health Protection Act (3) and the Comprehensive Environmental Response, Compensation and Liability Act of 1980, as amended by the 5Superfund Amendment and Reauthorization Act (4) of 1986.

The Toxicity Criteria Rule reduces uncertainty, time, and costs required to resolve different interpretations of federal guidance on applicable toxicity criteria, and disputes over which toxicity criteria to use at federal sites. It also assures consistent health protection across California.

To assist the public, DTSC developed the following brief answers to the most frequently asked questions regarding the Toxicity Criteria Rule.

  1. “Toxicity Criteria Rule”; Title 22, California Code of Regulations, sections 68400.5, 69020-69022.  (Effective date September 4, 2018). The regulatory text is also available at
  2. US Environmental Protection Agency, Risk Assessment Guidance for Superfund, EPA/540/1-89/002 (December 1989) Available at
  3. Children’s Environmental Health Protection Act, Senate Bill No. 25, Escutia, Chapter 1999, 1998-1999 Session.
  4. 42 U.S. Code sections 9601 et seq.
  6. The Toxicity Criteria Rule explicitly applies to all sites undergoing cleanup under the following laws: Hazardous Waste Control Act (Health and Safety Code section 25100 et seq., also known as “Chapter 6.5”), Hazardous Substances Account Act (Health and Safety Code section 25300 et seq. also known as “Chapter 6.8”), and the California Land Reuse and Revitalization Act (Health and Safety Code section 25395.60 et seq.).

What are toxicity criteria?

Toxicity criteria are chemical-specific numerical values that reflect the potency of a chemical or contaminant (5). There are two types of toxicity criteria: cancer and noncancer. Cancer toxicity criteria are health-protective values used to predict the increased cancer risk from a lifetime exposure to a chemical or contaminant of concern. The noncancer toxicity value is the amount of a chemical or contaminant that a person can ingest or breathe every day for a lifetime without any expected adverse health effects.

What does the Toxicity Criteria Rule do?

The Toxicity Criteria Rule specifies the toxicity criteria that must be used to prepare human health risk assessments and to calculate screening levels and remediation goals based on human health risk at California hazardous waste and hazardous substance release sites (6).  Additionally, the Rule defines cancer and noncancer risk-based screening levels of contaminants in a manner that’s consistent with federal guidance. Cancer risks for contaminants are set at one in a million (i. e., 1×10-6). Noncancer risks are set using a Hazard Index of 1. A hazard index of 1 or lower means estimated exposure to the contaminant at the screening level over a lifetime is not expected to cause harmful, noncancer health effects.

Is this Toxicity Criteria Rule changing DTSC’s methods for risk assessment?

No, the Toxicity Criteria Rule is consistent with how DTSC has selected and used toxicity criteria for the past 26 years.

How are the toxicity criteria used at California hazardous waste and hazardous substance release sites?

Risk assessors use toxicity criteria in the human health risk assessment process to calculate and assess health risk from exposure to contamination, and to determine if cleanup is needed at a hazardous waste or hazardous substance release site.

DTSC uses toxicity criteria to establish risk-based screening levels. After sufficient information is collected on a site to conduct a risk-based screening analysis, DTSC uses the result of a screening analysis to determine if site investigation or cleanup is needed. Consistent with federal and state guidance, the Toxicity Criteria Rule defines risk-based screening levels to be 1×10-6 (one in a million) excess lifetime cancer risk from exposure and a hazard index of “1” for a noncancer effect for each contaminant.  A hazard index of 1 or lower means estimated exposures over a lifetime to a chemical or mix of chemicals is not expected to cause adverse noncancer health effects. DTSC Human and Ecological Risk Office (HERO) Human Health Risk Assessment (HHRA) Note 3 provides DTSC-recommended screening levels based on toxicity criteria specified in the Toxicity Criteria Rule and DTSC’s recommended exposure parameters discussed in HHRA Note 1. Both Notes can be found at

DTSC also uses toxicity criteria as one of many factors in setting remediation goals (also referred to as cleanup levels) for a contaminated site. For more on this, see Question 13 below

What are DTSC’s primary sources of toxicity criteria?

The two primary toxicity criteria sources are the publicly peer-reviewed values from the:

  • California Office of Environmental Health Hazard Assessment (OEHHA) chemical database, and
  • U.S. Environmental Protection Agency (U.S. EPA) Integrated Risk Information System (IRIS) database.

Additional toxicity criteria sources include OEHHA toxicity criteria not listed in Toxicity Criteria Rule Appendix I (e.g., those used in U.S. EPA’s Regional screening levels), the U.S. Agency for Toxic Substances and Disease Registry Minimal Risk Levels, U.S. EPA’s Provisional Peer Reviewed Toxicity Values, and U.S. EPA’s Superfund Health Effects Assessment Summary Table Values.

How do I use the Toxicity Criteria Rule to identify the appropriate toxicity criteria for a contaminant?

Section 69021(a) directs users to first check Appendix I of the Toxicity Criteria Rule for the required value(s). If Appendix I does not provide toxicity criteria for the given contaminant and exposure pathway, then section 69021(b) directs users to the IRIS database.

If a value is also not available in the IRIS database, then section 69021(c) provides a list of other available sources of toxicity criteria as listed in Question 5 above. Please see Figure 1 below. Toxicity criteria from these or another health-based source that applies the best available science may be used if approved by the DTSC HERO Supervising Toxicologist. This has long been DTSC’s practice in the absence of peer-reviewed OEHHA and IRIS values.

To aid risk assessors in identifying an appropriate value under section 69021(c), HERO developed new HHRA Note 10, which discusses the process for selecting values and lists the most recently DTSC-approved toxicity criteria and their source.  HHRA Note 10 can be found at

Should a new IRIS value become available that is more protective than its counterpart in Appendix I, that new IRIS value must be used. For more information on this, please see FAQs 9 and 10 below.

The form or type of contaminant present at my site differs from the one on which the toxicity criteria are based. What value do I use?

Some contaminants present at a hazardous waste release site may be in a form or state different than that listed for the toxicity criteria. For example, metallic elements may exist in multiple oxidation states, form various organic and inorganic compounds, or be alloyed together with other metal or nonmetallic elements. The form, state, or compound may have a different toxicity than the elemental metals on which the toxicity criteria are based.

If the contaminant present at the site is in a different form or state than the metal listed in Appendix I, then use of Appendix I toxicity criteria may not be appropriate. In this case, the risk assessors evaluating the site should consult with the DTSC toxicologist on the appropriate toxicity criteria for the form or state of the metal present at the site to be sure that the toxicity criteria applied are based on the best available science, consistent with the requirements of H&SC 25356.1.5.

How do I determine appropriate toxicity criteria for total petroleum hydrocarbons?

Toxicity criteria for total petroleum hydrocarbons (TPH) mixtures fall under section 69021(c).  Petroleum is a mixture of various compounds that are often grouped and quantified as TPH.  Toxicity criteria developed for specific TPH compounds (e.g., benzene) are provided in Appendix I to the Toxicity Criteria Rule and the IRIS database. For each site, the project team should discuss and select the appropriate toxicity criteria for TPH mixtures using site-specific conditions (e.g., the sampling method utilized and the site-specific ratio of identified TPH constituents). Additional details are provided in HHRA Note 10, mentioned in Question 6 above.

How will the Toxicity Criteria Rule be updated to reflect new or updated toxicity criteria?

DTSC will periodically amend the Toxicity Criteria Rule to ensure that the most current, appropriate OEHHA toxicity criteria are listed in Appendix I (H&SC 25356.1.5). Additions or revisions to toxicity criteria values in Appendix I will be adopted through a formal or emergency rulemaking process consistent with the California Administrative Procedures Act (APA).  Among other conditions, DTSC will identify “a situation that calls for immediate action to avoid serious harm to the public peace, health, safety, or general welfare” to issue an emergency regulation. (Gov’t Code 11342.545) DTSC can use a time-limited, expedited rulemaking authority in an emergency. Under the non-emergency APA process, DTSC will:

  • Issue a formal notice and solicit comments on the proposed rulemaking. The rulemaking package will include the rationale for each proposed change and the technical documents explaining the derivation of those new values.
  • Seek public comment and hold a formal public hearing on the proposed rule.
  • Make appropriate changes to the proposal based on public comment.
  • Develop a final rule package that includes final responses to all comments on the proposed rule and submit it to the Office of Administrative Law for final approval and adoption.

What values should I use if new or updated toxicity criteria are available from OEHHA or IRIS before the Toxicity Criteria Rule is amended?

The Toxicity Criteria Rule specifies that if any new, more stringent IRIS value meets state statutory requirements, that new value will supersede a less protective, corresponding OEHHA value cited in Appendix I. As was done prior to the rule, DTSC reviews all new IRIS toxicity criteria. In consultation with OEHHA, DTSC’s HERO Supervising Toxicologist will determine whether the new IRIS value, or a corresponding OEHHA value meets the best available science requirements in H&SC section 25356.1.5. DTSC will then pursue an appropriate change to the rule, if needed, as well as approve the appropriate value for use in risk assessments, screening levels and remediation goals. Finally, if OEHHA issues a new toxicity value that meets statutory requirements (including the application of best available science) and is more stringent than the corresponding IRIS value, then DTSC recommends use of that new OEHHA toxicity criterion until the Toxicity Criteria Rule can be amended.

In all the above situations, risk assessors should monitor OEHHA and U.S. EPA notices to determine if new toxicity criteria must be used or will be part of a forthcoming Toxicity Criteria Rule amendment. To assist users in this, HERO publishes notices of revised recommended toxicity criteria through semiannual updates to HHRA Notes 3 and 10.

Will the Toxicity Criteria Rule impact previously approved remedies?

Adoption of the Toxicity Criteria Rule does not automatically change previously approved or selected remedies. Changes in toxicity criteria used for remedial decisions are normally considered if a Five-Year Review is conducted for the site; this is further discussed in Question12 below. For previously closed (e.g., certified) sites that are not subject to a Five-Year Review, DTSC may re-examine a closed site if there is evidence or substantial concern that the decision is not protective. Remedial action may be warranted should the evaluation determine the original decision is inadequately protective of human health and the environment. This is consistent with applicable hazardous substance and waste laws and guidance and the requirement that DTSC use the best available science in its risk assessments. If a site is re-opened to ensure the remedy protects public health and the environment, then additional analysis or remedial work may be needed, depending on site-specific factors and the revised remedial goals that apply. However, the Toxicity Criteria Rule itself does not require reopening of a final remedial decision as the rule does not apply more stringent standards than DTSC’s past practice for developing screening levels and remediation goals.

How does the Toxicity Criteria Rule affect Five-Year Reviews?

While federal and state guidance require that the Five-Year Review consider changes in toxicity criteria for contaminants still present at the site, the Toxicity Criteria Rule does not change future Five-Year Reviews because the Toxicity Criteria Rule does not change DTSC’s practice or standards for application of toxicity criteria; and the Appendix I criteria are the same as those previously recommended by DTSC. Also, the existing remedy may still be deemed protective because the toxicity criteria for a given site contaminant is only one of many factors that go into the determination of remedy protectiveness. Although a contaminant’s toxicity criteria may be updated to a more stringent value, that contaminant may not be driving the risk or remedial goal(s) at the site. Risk managers will continue to evaluate remedy protectiveness based on the risk management range and hazard based on site conditions as discussed in Question 14 below.

Is the Toxicity Criteria Rule consistent with federal guidance?

Yes. DTSC designed the Toxicity Criteria Rule to be consistent with related federal guidance.  The Toxicity Criteria Rule’s Final Statement of Reasons provides a detailed explanation of the Toxicity Criteria Rule’s consistency with federal guidance. In short, the Toxicity Criteria Rule is consistent with the most directly related guidance: U.S. EPA OSWER Directives 9285.7-16 and 9285.7-53, and U.S. EPA Risk Assessment Guidance Part A. The Toxicity Criteria Rule is also consistent with the Economic Council of States (ECOS) – Department of Defense Sustainability Work Group white paper. This ECOS white paper is not federal guidance, but does present an informal understanding by the military and member states, including California, of the selection process for toxicity criteria for hazardous waste and substance release sites. Each of these documents has a provision for individual states to apply credible toxicity criteria different than those in IRIS. Finally, federal laws and regulations allow states to apply scientifically credible levels that are more stringent than federal levels.

How to select toxicity criteria under the Toxicity Criteria Rule (1)

  • Consult Appendix I
    If toxicity criteria for the chemical and pathway is listed use that value.  If not, move to next step.
  • Consult USEPA IRIS Database
    If toxicity criteria for the chemical and pathway is listed use that value.  If not, move to next step.
  • Consult DTSC HHRA Note 10
    Use the DTSC pre-approved listed toxicity criteria for the chemical and pathway.

(1) Monitor HERO’s website for updates to Appendix I and HHRA Note 10.